The earthquake sensing device has become a popular appliance in the modern society. Especially when all kinds of safeguarding equipments have been invented to ensure the safety of home, industry, office and other public buildings, the earthquake sensing device has become a necessary component for a variety of equipments, instruments and tools.
An earthquake sensing device is designed to sense strong earthquakes and to generate warning signals so that the relative equipments, instruments and tools may take necessary protective actions accordingly.
Take computerized gas meter as an example. The measurement of gas flow rate in a computerized gas meter is conducted by a gas flow signal converting device. The flow rate signals are then processed by a microcomputer in order to calculate and to monitor the gas flow rate. In the gas meter, an earthquake sensing device is provided. The earthquake sensing device generates a warning signal to the microcomputer when a strong earthquake is sensed. The microcomputer generates a control signal to a shutoff mechanism to stop the gas supply, when a warning signal is generated by the earthquake sensing device. With this facility, fire accidents which always take place after a strong earthquake may be avoided.
In addition, in the fine equipments and instruments used in the industry or in the laboratories, errors in manufacture, control or measurement will appear due to the influence of earthquakes. An earthquake sensing device is useful to indicate possible influences of earthquakes so that proper calibration may not be omitted.
Due to the importance of the earthquake sensing device, a variety of high precision, compact and low-cost earthquake sensing devices are being developed.
U.S. Pat. No. 5,408,457 (assigned to Osaka Gas Co., Ltd. and Kansai Gas Meter Co., Ltd.) disclosed an earthquake sensing device for use inside a gas meter. The earthquake sensing device is capable to sense strong earthquakes and comprises a steel ball hung inside a space surrounded by a continuous wall. When an earthquake takes place, the movement of the steel ball will cause a series of ON and OFF signals generated by an inductive circuit located above the steel ball. This ON/OFF signals may be used to determine the existence of an earthquake. The advantage of this design is that such earthquake sensor is simple and consumes very little electric power. Its disadvantage, on the other hand, is that it is not capable to measure the magnitude of an earthquake correctly. As a result, it is necessary to provide an additional logic circuit to determine whether a warning signal should be generated.
Japan Patent Publication No. 07-020248 disclosed an earthquake magnitude determination logic applicable to said U.S. Pat. No. 5,408,457. The determination logic of this patent includes a logic trueth table wherein values of features of an earthquake such as total period of earthquake, number of cycles of shakes and time of shaking cycles are taking as factors. The advantage of this patent is that the truth table may be used to design the control circuit. Its disadvantage, however, is that the truth table is not applicable to areas where patterns of earthquake are different.
In addition to the above, in Douglas P. Ardunini's article "Smart Sensor Requirements for Second Generation Seismic Gas Shut-Off Valves" and in John Andrew Miche's U.S. Pat. No. 5,742,235, some features of earthquake were disclosed. According to their analysis, in an earthquake, its vertical waves (p-waves) will have a higher transmission speed than that of the horizontal waves (s-waves). On the other hand, the magnitude and the destroying power of the vertical wave are far smaller than that of the horizontal waves. As a result, in said U.S. Pat. No. 5,742,235, a micro vibration switch made from a semiconductor manufacture process was invented to sense the vertical waves of an earthquake. The advantage of this invention is that the structure of this earthquake sensor is simple and that its manufacture cost is relatively low. However, according to another analysis of the features of earthquakes, the magnitude of the vertical waves of an earthquake is very tiny (about 0.01 g). Under such a tiny scale, errors in the determination of an earthquake are easily caused, if only the vertical waves are sensed and are used as the only basis of the determination.
It is thus a need in the industry to provide a high-precision, compact and low-cost earthquake sensor.